Because useful information concerning the composition of a liquid can be derived by measuring its conductivity, conductivity measuring cells have come into widespread use as analytical tools. Conductivity cells include a pair of electrodes for immersion in a body of liquid of known size and shape, and are used by applying a known voltage and measuring the resulting current. Equivalently, a known current may be established between the electrodes and the resulting voltage measured.
Because test liquids may have a wide range of conductivities, it is customary to use a conductivity cell with an electrode length and spacing that is appropriate for the expected range of conductivity for the liquid. In liquids that have high resistances (i.e., low conductivities), for example, it is desirable to have the electrodes project deeply into the solution so that the flow of current therebetween has a conveniently measurable magnitude. With liquids that have low resistances (i.e., high conductivities), on the other hand, it is desirable to have the electrodes project less deeply into the solution so that the flow of current therebetween again has a conveniently measurable magnitude. A conductivity cell of the former type is said to have a low cell constant, while a cell of the latter type is said to have a high cell constant.
While the possibility of making the cell constant adjustable by providing an adjustable electrode length would appear to be attractive, it is not in many cases economical or convenient to do so. One reason is that the conductivity of a liquid is a nonlinear function of the exposed electrode length due to fringing and other effects. This nonlinearity makes it necessary either to provide an adjustment arrangement which takes into account this nonlinearity, or to restandardize the cell after each change in electrode length. As a practical matter, therefore, it is more economical and convenient to have a number of conductivity cells with different fixed cell constants and to change cells when liquids having different ranges of conductivity are to be measured.
While the above-described way of dealing with the need for differing cell constants simplifies the task of the cell user, it complicates the task of the cell manufacturer. The reason is that, prior to the present invention, manufacturers have been able to mount electrodes in cell housings in one of two basic ways. Firstly, the manufacturer could premold the electrodes into a molded subassembly that provided the desired electrode length and/or spacing and then mount this subassembly into a cell housing of a common, standard design. This approach is relatively costly because it involves the cost of producing a number of different molds and molded subassemblies for the different electrode configurations, and the cost of the structures and parts necessary to align and retain the different electrode subassemblies within the cell housing. Secondly, the manufacturer could mold a number of different cell housings in which electrodes could be potted to provide the desired cell constants. This approach is also relatively costly because of the investment necessary to produce the molds for a number of different housings. In addition, the potting compound often leaked onto or wetted parts of the electrodes that must be bare metal, causing the cell constant the deviate from the desired value.